In recent years, an optical disk such as CD and DVD has widely been used as an information recording medium.
The information recording medium can be roughly divided into three types: a read only type called ROM, a write once type called R, and a rewritable type called RW.
Also, the recording and reproducing appararus for the recording medium has been developed and significantly popularized.
In the information recording medium, an optical disk includes at least the following characteristics (1)–(4):
(1) Compared to a magnetic tape medium, optical disk provides a better access performance to information;
(2) Compared to a semiconductor memory, recording capacity per unit is larger, and low unit price of capacity can be realized;
(3) It is possible to separate the optical disk from an apparatus itself and to handle the optical disk indivisually; and
(4) It is possible to preserve recorded information in a good condition.
The above-mentioned characteristics (1) and (2) also correspond to those of hard disk apparatus, and the hard disk apparatus is widely recognized and popularized for the use of temporarily storing information.
The reason an optical disk and an optical disk apparatus have been developed and popularzed separately from a hard disk apparatus is mainly because of the above characteristics (3) and (4).
An optical disk is the most suitable information recording medium for storing a large volume of information such as movie and music, which allows one to buy and sell a medium itself.
An optical disk is also suitable for carefully recording and storing information which has been produced personally, and in case of a shortage of capacity, it is possible to increase the capacity for storing information only by making an additional medium, which can be obtained at low cost as noted in the above characteristics (2).
An optical disk has not only these good characteristics but also some problems.
That is, information recorded on the optical disk is digital information. Unlike analog information, digital information, which is re-recorded to another digital recording apparatus, in principle does not deteriorate with respect to original information. This means that it is possible to copy as much information which is the same as the original information as one likes.
Meanwhile, some optical disks in which movie and music are stored include copyrighted image and audio information. Unauthorized copying of the optical disk including such copyrighted information to other inexpensive recording medium, that is, distribution of a pirated edition to the public, is considered as an infringement of copyright.
As methods for protecting copyright with respect to the optical disk, the following techniques (1)–(3) are known.
(1) For example, CSS (Content Scrambling System), a copyright protection system using the encryption technique, is selected for DVD. Encrypting copyrighted information to be recorded on the optical disk prevents unauthorized reproduction of information.
(2) Data is recorded on the optical disk in two different pit depths, so that information based on a quantity of reflected light from the pit and information based on the polarity of tangential push/pull signal from the pit are obtained (Japanese Laid-Open Patent Publication by the present applicant: JP-A-2001-76347, published on Mar. 23, 2001).
Even if pit information of this optical disk is copied, information based on the polarity of tangential push/pull signal can not be copied physically to other recording medium.
The reason for this is that the tangential push/pull signal is generated by physically changing the pit depth, making it impossible for other recordable medium (magnetic recording medium or phase change recording medium) to record it.
Therefore, recording copyright over the information based on the polarity of tangential push/pull signal makes it possible to set reproducing restriction on the recording medium from which the information cannot be detected.
(3)Digital watermarking technique is recently focused on. The digital watermarking technique has been introduced, for example, in NIKKEI ELECTRONICS (issued on Feb. 24, 1997, p149-p162).
In digital watermarking technique, copyright information can be embedded in copyrighted information. The embedded copyright information, even if it is copied, remains in information copied by unauthorized copying, thereby finding whether or not the information is the subject of copyright protection.
However, the prior art described above has the following problems.
In the copyright protection technique described in the above (1), at the stage of decrypting the encryption, original copyrighted information can be read out.
Many users watch DVD on a recent personal computer, and in such case, decryption is often performed on the CPU of the personal computer with software.
Thus, it is possible for one who has some level of knowledge and technique to decrypt encrypted copyrighted information and to read it out as digital information by tempering with the software.
By copying the digital information to other recordable recording medium such as an optical disk, a pirated edition, which can be reproduced in a common reproducing apparatus, is produced.
In the technique using the tangential push/pull signal described in the above (2), it is possible to set reproducing restriction on the optical disk from which information based on the tangential push/pull signal cannot be detected, that is, the optical disk assumed to have been copied by unauthorized copying.
However, this means that the reproducing restriction is also set on the optical disk which does not include information based on the polarity of tangential push/pull signal (optical disk in which copyright-free information is recorded, write once disk, and rewritable disk).
In addition, in the technique (2) described above, it is impossible to prevent unauthorized copying of data that was created, for example, by encoding images delivered on a screen.
In the digital watermarking technique described in the above (3), the copyright information by digital watermarking remains in the disk of pirated edition.
Therefore, it is possible to detect whether or not the disk of pirated edition includes copyrighted information.
However, while this technique may be able to warn the user, it is not capable of preventing copying to other recording medium. Therefore, re-distribution of the disk of pirated edition to the public is possible, and the product disk of pirated edition can still be reproduced by a common reproducing apparatus.
In addition, in the technique (3) described above, if a measure is taken to prohibit recording and reproducing of the recording medium in which copyright information is not embedded, it becomes impossible to reproduce the recording medium which is not subject to copyright protection.